I think only console versions can be played offline, the reason for always online approach is to avoid cheaters/duping itens and ofcourse to avoid pirate copies.
In my opinion Diablo III with the Reaper of Souls expansion is better than the legendary Diablo II; Blizzard screwed in the initial release but fixed almost all problems later and improved a lot with the expansion.
The latency problem seems to be fixed as well:
It is a great game for people who like to "grind", amazing longevity that will entertain for years to come.